CI: Python 3.10.0-rc.2 coverage fix

[FollowTheProcess]

Closes #478

After some playing around with my fork I've managed to get tests and coverage playing nicely on 3.10.

Summary of changes:

• Detect when we're running on 3.10 and reduce coverage fail under to 99 from 100
• Parametrise cover with the same pythons as tests so I could access session.python to do the above
• Explicitly call cover-<version> in the Github Actions workflow (session.notify didn't seem to work for me here, I tried both "cover" and "cover-<version>" as arguments with no luck, if anyone knows how to fix this it would clean this solution up a little bit)

I did also notice that one of the tests (test_session_completer) was coupled to the noxfile. I didn't look into it too much but I think we should look at decoupling if possible as then we're free to change the noxfile however we want without altering test behaviour

[cjolowicz]

I did also notice that one of the tests (test_session_completer) was coupled to the noxfile. I didn't look into it too much but I think we should look at decoupling if possible as then we're free to change the noxfile however we want without altering test behaviour

Agreed 👍

[cjolowicz]

Thanks 🚀

[FollowTheProcess]

Thanks 🚀

Thanks for the help as always!